MRI is an effective, non-invasive magnetic imaging technique for generating sharp images of the internal anatomy of the human body, which provides an efficient means for diagnosing disorders such as neurological and cardiac abnormalities and for spotting tumors and the like. Briefly, the patient is placed within the center of a large superconducting magnetic that generates a powerful static magnetic field. The static magnetic field causes protons within tissues of the body to align with an axis of the static field. A pulsed RF magnetic field is then applied causing the protons to begin to precess around the axis of the static field. Pulsed gradient magnetic fields are then applied to cause the protons within selected locations of the body to emit RF signals, which are detected by sensors of the MRI system. Based on the RF signals emitted by the protons, the MRI system then generates a precise image of the selected locations of the body, typically image slices of organs of interest.
However, MRI procedures are problematic for patients with implantable medical devices such as pacemakers and ICDs. One of the significant problems or risks is that the strong RF fields of the MRI can induce currents through the lead system of the implantable device into the tissues, resulting in Joule heating in the cardiac tissues around the electrodes of leads and potentially damaging adjacent tissues. Indeed, the temperature at the tip or ring of an implanted lead has been found to increase as much as 60° for tip or 20° for ring Celsius (C.) during an MRI tested in a gel phantom in a non-clinical configuration. Although such a dramatic increase is probably unlikely within a clinical system wherein leads are properly implanted, even a temperature increase of only about 8°-13° C. might cause myocardial tissue damage.
Furthermore, any significant heating of cardiac tissues near lead electrodes can affect the pacing and sensing parameters associated with the tissues near the electrode, thus potentially preventing pacing pulses from being properly captured within the heart of the patient and/or preventing intrinsic electrical events from being properly sensed by the device. The latter might result, depending upon the circumstances, in therapy being improperly delivered or improperly withheld. Another significant concern is that any currents induced in the lead system can potentially generate voltages within cardiac tissue comparable in amplitude and duration to stimulation pulses and hence might trigger unwanted contractions of heart tissue. The rate of such contractions can be extremely high, posing significant clinical risks to patients. Therefore, there is a need to reduce heating in the leads of implantable medical devices, especially pacemakers and ICDs, and to also reduce the risks of improper tissue stimulation during an MRI, which is referred to herein as MRI-induced pacing.
Various techniques have been developed to address these or other related concerns. See, for example, the following patents and patent applications: U.S. patent application Ser. No. 11/943,499, filed Nov. 20, 2007, of Zhao et al., entitled “RF Filter Packaging for Coaxial Implantable Medical Device Lead to Reduce Lead Heating during MRI”; U.S. patent application Ser. No. 12/117,069, filed May 8, 2008, of Vase, entitled “Shaft-mounted RF Filtering Elements for Implantable Medical Device Lead to Reduce Lead Heating During MRI”; U.S. patent application Ser. No. 11/860,342, filed Sep. 27, 2007, of Min et al., entitled “Systems and Methods for using Capacitive Elements to Reduce Heating within Implantable Medical Device Leads during an MRI”; U.S. patent application Ser. No. 12/042,605, filed Mar. 5, 2009, of Mouchawar et al., entitled “Systems and Methods for using Resistive Elements and Switching Systems to Reduce Heating within Implantable Medical Device Leads during an MRI”; and U.S. patent application Ser. 11/963,243, filed Dec. 21, 2007, of Vase et al., entitled “MEMS-based RF Filtering Devices for Implantable Medical Device Leads to Reduce Lead Heating during MRI.”
See, also, U.S. patent application Ser. No. 12/257,263, filed Oct. 23, 2008, of Min, entitled “Systems and Methods for Exploiting the Tip or Ring Conductor of an Implantable Medical Device Lead during an MRI to Reduce Lead Heating and the Risks of MRI-Induced Stimulation; U.S. patent application Ser. No. 12/257,245, filed Oct. 23, 2008, of Min, entitled “Systems and Methods for Disconnecting Electrodes of Leads of Implantable Medical Devices during an MRI to Reduce Lead Heating while also providing RF Shielding”; and U.S. patent application Ser. No. 12/270,768, of Min et al., filed Nov. 13, 2008, entitled “Systems And Methods For Reducing RF Power or Adjusting Flip Angles During an MRI For Patients with Implantable Medical Devices.”
At least some of these techniques are directed to installing RF filters, such as inductive (L) filters or inductive-capacitive (LC) filters, within the leads for use in filtering signals at frequencies associated with the RF fields of MRIs. It is particularly desirable to select or control of the inductance (L), parasitic capacitance (Cs) and parasitic resistance (Rs) of such devices to attain a high target impedance (e.g. at least 1000 ohms) at RF to achieve effective heat reduction. See, for example, U.S. patent application Ser. No. 11/955,268, filed Dec. 12, 2007, of Min, entitled “Systems and Methods for Determining Inductance and Capacitance Values for use with LC Filters within Implantable Medical Device Leads to Reduce Lead Heating During an MRI; and U.S. patent application Ser. No. 12/325,945, of Min et al., filed Dec. 1, 2008, entitled “Systems and Methods for Selecting Components for Use in RF Filters within Implantable Medical Device Leads based on Inductance, Parasitic Capacitance and Parasitic Resistance.”
Although these techniques are helpful in reducing lead heating due to MRI fields, there is room for further improvement. In particular, it would be desirable to provide RF filtering without requiring one or more discrete or lumped L or LC filters, as such filters can be harder to be implemented in a limited space allowed in a lead and to meet required mechanical reliability. One possible solution is to provide for some form of distributed inductance along the length of the lead. However, problems arise in providing distributed inductance along medical device leads, particularly the leads of pacemakers and ICDs.
One such problem is due to the “coiling effect.” It has been found that any coiling of excess lead length by the clinician during device implant can affect the amount of heat reduction achieved using distributed RF filtering elements. In this regard, following implant of the distal ends of leads into heart chambers, and prior to connection of the proximal ends of the leads into the pacemaker or ICD being implanted, there may be some excess lead length. Clinicians often wrap or coil the excess lead length around or under the pacemaker or ICD prior to connecting the leads to the device. This can negate the efficacy of heat reduction features in leads, particularly the efficacy of distributed inductive filtering components, potentially resulting in an increase of over 30° C. as compared to leads not coiled around or under the device. This interference in heat reduction caused by wrapping the lead around or under the device is referred to herein as the coiling effect.
It is believed that the coiling effect may be due to shunt capacitance between the proximal portions of the lead that are wrapped around or under the device and the housing of the device itself (particularly when proximal portions of the leads include some form of inductive filtering element) as well as changes related to loops (such as impedance/phase changes at the location of the end of coiling section.) As noted, a high target impedance at RF is desired to reduce heating due to the RF fields of the MRI. Insofar as leads with distributed inductive components are concerned, the actual impedance achieved depends, in part, on the inductance (L) and the parasitic capacitance and resistance (Cs, Rs) of the components distributed along the lead. Coiling the lead around or under a device appears to reduce the inductance of insulated coils and also add a shunt capacitance between the distributed components along the proximal end of the lead and the metallic case of the device, which adversely affects the resulting L, Cs and Rs values and hence allows for greater unwanted heating during MRIs when the performance depends on accumulated effect of distributed insulated coils.
So, one concern with implementing distributed inductive RF filters within leads is shunt capacitance due to the coiling effect (particularly involving any distributed components mounted along the proximal end of the lead.). This can be related to lead length if the shortest possible lead length in clinical setting (e.g. 25 cm or shorter) does not meet impedance requirements for RF heating reduction.
Accordingly, it would be desirable to provide improved lead designs that achieve heat reduction during MRIs without requiring conventional discrete or lumped RF filtering components and without requiring otherwise conventional distributed filtering components. Various aspects of the invention are directed to this end.